As easy-to-access and easy-to-produce hydrocarbon resources are depleted, there is an increased demand for more advanced recovery procedures. One such procedure is steam assisted gravity drainage (SAGD), a procedure that utilizes steam in conjunction with two spaced apart wellbores. Specifically, SAGD addresses the mobility problem of heavy oil in a formation through the injection of high pressure, high temperature steam into the formation. This high pressure, high temperature steam reduces the viscosity of the heavy oil in order to enhance extraction. The injection of steam into the formation occurs from a first wellbore (injector) that is drilled above and parallel to a second wellbore (producer). As the viscosity of the heavy oil in the formation around the first wellbore is reduced, the heavy oil drains into the lower second wellbore, from which the oil is extracted. Preferably, the two wellbores are drilled at a distance of only a few meters from one other. The placement of the injector wellbore needs to be achieved with very small margin in distance. If the injector wellbore is positioned too close to the producer wellbore, the producing wellbore would be exposed to very high pressure and temperature. If the injector wellbore is positioned too far from the producer wellbore, the efficiency of the SAGD process is reduced. In order to assist in ensuring that the second wellbore is drilled and positioned as desired relative to the first wellbore, a survey of the two wellbores in the formation is often conducted. These surveying techniques are traditionally referred to as “ranging”.
One solution that has been employed in ranging is to use ranging devices to directly sense and measure the distance between two wells as the latter wellbore is drilled. Two wellbore-known commercial approaches that employ equipment in both wells (injector and producer) are based either on rotating magnets or magnetic guidance techniques. However, these approaches are undesirable in that they require two separate and different teams to manage the equipment in each wellbore, namely, a wireline team at the producer wellbore and a logging while drilling team at the injector wellbore, which is not cost effective. One prior art approach utilizes equipment in only a single wellbore (the injector wellbore) to transmit a current to a target wellbore (the producer wellbore), after which an absolute magnetic field measurement is used to calculate distance.